Responses to elevated CO"2 have not been measured for natural grassland ecosystems. Global carbon budgets will likely be affected by changes in biomass production and allocation in the major terrestrial ecosystems. Whether ecosystems sequester or release excess carbon to the atmosphere will partly determine the extent and rate that atmospheric CO"2 concentration rises. Elevated CO"2 also may change plant community species composition and water status. We determined above- and belowground biomass production, plant community species composition, and measured and modeled water status of a tallgrass prairie ecosystem in Kansas exposed to ambient and twice-ambient CO"2 concentrations in open-top chambers during the entire growing season from 1989 through 1991. Dominant species were Andropogon gerardii, A. scoparius, and Sorghastrum nutans (C"4 metabolism) and Poa pratensis (C"3). Aboveground biomass and leaf area were estimated by periodic sampling throughout the growing season in 1989 and 1990. In 1991, peak biomass and leaf area were estimated by an early August harvest. Relative root production among treatments was estimated using root ingrowth bags which remained in place throughout the growing season. Latent heat flux was simulated with and without water stress. Botanical composition was estimated annually. Compared to ambient CO"2 levels, elevated CO"2 increased production of C"4 grass species, but not of C"3 grass species. composition of C"4 grasses did not change, but Poa pratensis (C"3) declined, and C"3 forbs increased in the stand with elevated CO"2 compared to ambient. Open-top chambers appeared to reduce latent heat flux and increase water-use efficiency similar to the elevated CO"2 treatment when water stress was not severe, but under severe water stress, the chamber effect on water-use efficiency was limited. In natural ecosystems with periodic moisture stress, increased water-use efficiency under elevated CO"2 apparently would have a greater impact on productivity irrespective of photosynthetic pathway.

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A tallgrass prairie ecosystem was exposed to ambient and twice‐ambient CO2 concentrations in open‐top chambers and compared to unchambered ambient CO2 during the entire growing season from 1989 through 1991. Dominant species were Andropogon gerardii (C4), A. scoparius (C4), Sorghastrum nutans (C4) and Poa pratensis (C3). Nitrogen and phosphorus concentrations in A. gerardii, P. pratensis and dicotyledonous herbs above ground biomass were estimated by periodic sampling throughout the growing season in 1989 and 1990. In 1991, N and P concentrations in peak biomass were estimated by an early August harvest. N and P concentrations in root production as a function of treatment were estimated using root ingrowth bags that remained in place throughout the growing season. Total N and P in above‐ and belowground biomass were calculated as products of concentration and peak biomass by species groups. N concentration in A. gerardii and dicotyledonous herb aboveground biomass was lower and total N higher in elevated CO2 plots than in ambient CO2 plots. N concentration in P. pratensis aboveground biomass was lower in elevated CO2 plots than in ambient, but total N did not differ among treatments in 2 out of 3 years. In 1990, N concentration in root ingrowth bag biomass was lower and total N greater in elevated CO2 than in ambient CO2 plots. Root ingrowth bag biomass N concentration did not differ among treatments in 1991, but total N was greater in elevated CO2 plots than in ambient CO2 plots. P concentration was lower under elevated CO2 compared to ambient in 1989, but did not differ substantially among treatments in 1990 or 1991. In all years, total P in aboveground A. gerardii and root ingrowth bag biomass was greater under elevated CO2 than ambient. P concentration and total P in P. pratensis was similar among treatments.

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